This invention relates to solid state laser resonators and more particularly to a solid state laser resonating cavity in which the path of a lasing beam comprises at least three points of reflection which are confined to a single plane. The invention may be used to produce single-longitudinal mode unidirectional laser radiation, which has wide applications.
It is often desirable to have a laser which lases single frequency. When the solid state lasant is a homogeneously-broadened material such as Nd:YAG, single longitudinal mode oscillation should, in theory, be readily achieved when continuous wave (CW) pumping is employed. In practice, single longitudinal mode lasing is difficult to achieve in conventional lasers. Conventional linear lasers consist of two reflecting mirrors forming a linear resonator producing standing waves as the normal modes of oscillation. At the nodes of a given standing wave the gain is not saturated, allowing one or more other standing waves of different frequency to access this gain and be above threshold for lasing. This is termed "spatial hole burning".
Another type of laser is the ring laser. A characteristic of a ring laser which differentiates it from a conventional linear laser is the ability to lase in a single direction (unidirectional). When a ring laser formed by at least three reflective mirrors is caused to lase unidirectionally, the normal mode of oscillation is a travelling wave whose path forms a polygon (ring) dictated by the number and configuration of mirrors in the resonator. In contrast to a standing wave, the travelling wave saturates the gain uniformly and the ring resonator lases in a single longitudinal mode as predicted.
Ring lasers of the prior art normally support counter-propagating oscillations. Such counter-propagating waves can interfere with each other, resulting in spatial hole burning as per conventional linear lasers.
It is well known that ring lasers can be made to lase unidirectionally by introducing some mechanism for causing one propagation direction to have less round-trip loss than the other. However, such a round-trip loss differential for unidirectional CW lasing is typically very small (&lt;1%). The standard technique for generating such a differential loss is to combine within the ring laser three distinct elements: nonreciprocal polarization rotation, reciprocal polarization rotation, and a polarizer. In one direction of propagation the rotation from reciprocal polarization rotation and nonreciprocal polarization rotation are additive. This larger rotation results in a linear polarization mode which suffers high loss at the polarizer. In the opposite direction of propagation the rotation from reciprocal polarization and nonreciprocal polarization rotations subtract resulting in a polarization mode which has lower losses at the polarizer.
In the standard art, nonreciprocal polarization rotation is achieved by placing a magneto-optic element in the presence of a strong magnetic field. The effect is known as the Faraday Effect. Reciprocal polarization rotation is commonly achieved with a linear birefringent waveplate of suitable phase retardation.
Nilsson et al., SPIE Proceedinqs, Vol. 912, p. 13 (Jan. 1988), has described the special aspects of unidirectional ring lasers which allow them to exhibit reduced effects caused by scattered output light (feedback) from external objects, such as uncoated lenses and optical fibers. Feedback into conventional lasers is a destabilizing effect resulting in frequency and amplitude fluctuations.
Practical implementation of a conventional ring laser results in a resonator consisting of numerous parts which is difficult to align and keep aligned. In addition to such long term mechanical instability, fast relative motions of the ring laser resonator mirrors manifest themselves as rapid frequency fluctuations, limiting the frequency stability of the ring laser. It is desirable to reduce the number of elements required to achieve unidirectional lasing to ideally a single piece of solid state laser material, such single piece solid state laser being termed monolithic. The monolithic laser is preferably configured so as to incorporate all elements responsible for unidirectional lasing.
Kane and Byer have disclosed a "Solid-State Non-Planar Internally Reflecting Ring Laser" in U.S. Pat. No. 4,578,793, issued Mar. 25, 1986. The apparatus described therein is a monolithic ring resonator fabricated as a nonplanar internally-reflecting prism. Such a monolithic ring laser incorporates at least four mirrored surfaces to define a ring ray path which travels along two distinct planes. This nonplanarity serves to generate reciprocal polarization rotation within the monolithic ring resonator. By locating the monolithic resonator within a strong magnetic field and choosing a solid state lasing material possessing finite magneto-optic properties, nonreciprocal Faraday Effect polarization rotation takes place within the monolithic resonator itself. Non-normal incidence of the internal ray path at the dielectric coated output mirror serves as a partial polarizer. Such monolithic nonplanar ring lasers demand adherence to strict dimensional and angular fabrication tolerances. Moreover, the nonplanar nature of this monolithic ring resonator requires numerous fabrication steps, making high volume manufacture costly. Kane has suggested that, although a planar ring laser would alleviate such difficulties, a planar unidirectional monolithic ring oscillator is not possible. (See Ph.D Dissertation, Stanford University, 1986.)
Another nonplanar design is described in Trutna et al., "Unidirectional diode-laser-pumped Nd:YAG ring laser with a small magnetic field," Optics Letters, Vol. 12, No. 4, pp. 248-250, Apr. 1987. The Trutna paper described a nearly planar design to better match the reciprocal and nonreciprocal polarization rotations.
It is known in the prior art, as for example, Kubodera et al., Appl. Opt. 18, 3882 (1979), that axial pumping with a laser is a means for achieving single transverse mode operation in solid state lasers. The laser pump light is conditioned so as to excite a region of the laser material totally contained within the TEM.sub.00 mode volume. This causes the TEM.sub.00 mode to access the available gain more efficiently than other higher order transverse modes. The TEM.sub.00 mode saturates the gain, and such a laser is "gain bound" to lase in a single transverse mode. This is in contrast to conventional lasers, which pump the entire laser material and use a lossy aperture to suppress higher order transverse modes which would otherwise lase due to the large pumped volume. Axial pumping provides a means to preferentially excite the TEM.sub.00 mode, without the negative effects on efficiency associated with lossy apertures.
Sipes, Jr., U.S. Pat. No. 4,710,940, has disclosed a method of pumping a plurality of lasing media, each with individual laser diode pumps and of providing a means for coupling the individual lasing media together in a unidirectional ring laser. The multiplicity of individual elements give rise to concerns about stability and other limitations known to conventional ring lasers. FIG. 18 represents the prior art of Sipes, Jr., for axially pumping a unidirectional Nd:YAG ring laser. Gain media L1, L2, L3 and L4 are coupled together by ring resonator mirrors M1, M2, M3 and M4. Mirrors M1, M2 and M4 are highly reflective at the 1064 nm lasing wavelength of Nd:YAG and transmissive at the wavelength of laser diode pump sources P1, P2, P3, P4, P5 and P8. Mirror M3 is the output coupler of the ring resonator and is also transmissive to pump sources P6 and P7. Mirrors M5 and M7 are high reflectors at 1064 nm and transmissive at the pump wavelength. Unidirectional lasing output 40 is meant to occur due to injection of 1064 nm laser light from InGaAlAs laser diode 30. No solid state resonator structure suitable for use in this configuration has been suggested.
It has been reported that it is often desirable to rapidly tune the frequency of the ring laser output in a controlled fashion. Prior art frequency tuning of monolithic lasers has included temperature, piezoelectric and magnetic tuning. T. J. Kane and R. L. Byer, Opt. Lett. 10, 65 (1985); A. Owyoung and P. Esherick, Opt. Lett. 12, 999 (1987); and T. J. Kane and E. A. P. Cheng, Opt. Lett. 13, 970 (1988). Piezoelectric tuning has been the preferred method for achieving fast tuning rates. It is therefore desirable for any new ring laser to incorporate tuning means which are at least as fast and practical to implement as those currently known.